Method and apparatus for irradiation welding of two thermoplastic components

ABSTRACT

In a method and an apparatus for irradiation welding of two thermoplastic components, a local temperature maximum, which circulates together with the laser beam and increases from cycle to cycle, and a correspondingly circulating, locally increased clamping pressure are generated in the area of joining.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of irradiation welding of twothermoplastic components by producing a weld seam in an area of joiningbetween the absorptive and transmissive component parts by means of anenergy beam, in particular laser beam. The invention further relates toan apparatus for putting the method into practice.

2. Background Art

For better understanding of the invention, the fundamentals of laserirradiation welding of plastics are going to be explained, taken inconjunction with prior art systems.

In laser irradiation welding of plastics, laser irradiation penetratesthe first weld seamed part that is turned towards the beam source and isbeing absorbed by the second weld seamed part by only minor depth ofpenetration of the surface and converted into heat. By heat conductionthe transmissive weld seamed part is equally being melted.

In so-called quasi simultaneous welding, the laser beam is being runrapidly along the weld contour for several times. Quasi simultaneouswelding of plastic components has established as a very common method inlaser irradiation welding. U.S. Pat. No. 6,444,946 B1 describes acorresponding method, specifying that the entire weld seam is beingplasticized substantially in a single cycle after being heated to an“intermediate temperature” by a plurality of cycles. The principal ideaof this method consists in the weld seam being heated as uniformly aspossible; any spatial temperature gradients along the weld seam are notdesired.

According to this document, in case of a closed weld seam, all areas ofthe weld seam are in a solid phase or all sectional areas aresimultaneously in a plasticized condition, uniform melting of the seamtaking place by the action of the joining pressure.

This welding method has the following drawbacks:

-   -   Current examinations of the working mechanism of plastics        welding have shown that high clamping pressure positively        affects the welding result. The assumption is that high contact        periods of the parts being joined, accompanied with        correspondingly high pressure, work in favour of molecular        exchange processes (diffusion). Additionally, high clamping        pressures improve the thermal contact of the parts being joined        and accelerate the heat conduction into the top layer. The load        that may act on a component part is as a rule limited, because        damages of the part will produce easily. With the clamping force        that is applied in a joining method according to U.S. Pat. No.        6,444,946 B1 spreading uniformly across the entire weld seam,        given the fact that, without obstruction to expansion, the weld        seam would stay nearly level by simultaneous plasticizing and        the inferior temperature gradients, the locally produced        clamping pressure decreases. With the effects of process        acceleration of the high clamping pressure not being exploited,        the efficiency, and thus economic profitability, of the welding        method decrease.    -   In connection with quasi simultaneous welding, so-called weld        seam-run monitoring is frequently used as a method of process        diagnostics. In this case, the length is measured, by which the        parts move towards one another under the action of joining        pressure and with the seam plasticized. As a rule, this takes        place by way of detection of the motion of the movable clamping        plate. Drawbacks reside in that weld seam interruptions cannot        be detected, because they do not affect the motion of the        clamping plate, provided the spatial expansion of seam        interruption is insignificant as compared to the entire seam        contour, which is the rule.    -   Heat conduction, which confers energy from the absorbtive part        being joined on the transmissive part being joined, is strongly        time-dependent. For the transmissive top layer to melt, which is        necessary for integral material joining, both parts being joined        must stay in thermal contact for a certain time. During this        time, due to the nearly uniform plasticizing of the weld seam,        molten polymer compound is displaced from the area of joining by        the joining pressure. The result is undesired escape of energy        from the area of welding that comprises the heated polymer        compound, which is accompanied with reduced process efficiency.        Additionally, displaced polymer compound may flow into component        areas where it is not desired for optical or functional reasons.    -   Notches in the absorbtive part being joined negatively affect        the welding process. The notch can only be filled with polymer        compound when the adjacent areas have been plasticized and a        sufficient amount of polymer compound has been displaced for the        notch to be closed. Only then the transmissive joining part        above the notch can be supplied with energy by thermal contact        via heat conduction. In addition to this negative effect, the        polymer at the bottom of the notch can be damaged thermally,        because it is heated as are intact areas of the weld seam seam,        while however only little thermal energy is led by heat        conduction to the cold top layer.

In contour welding the weld seam is being plasticized locally, theintegral joint occurring directly after a single irradiation job.Consequently this process is accompanied with some restrictions too:

-   -   There is no setting motion towards each other of the two parts        being joined, which is why any geometric compensation for        component tolerances is not possible.    -   Since the top layer of the joint is being plasticized mainly by        heat conduction from the bottom layer, integral joining will        take a certain time. For the polymer of the absorbtive bottom        layer not to be damaged, the maximally employable laser energy        density is limited upwards. Both circumstances explain why only        inferior feed velocities can be used in contour welding, which        is accompanied with prolonged process times than in quasi        simultaneous welding.

SUMMARY OF THE INVENTION

Proceeding from the described problems of prior art irradiation methods,it is an object of the invention to improve the corresponding methodsand apparatuses in such a way that simple means, in terms of the methodand apparatus, will do for rendering a welding process more reliable andefficient, offering safe results and compensating for any componentflaws.

Accordingly, the method of irradiation welding according to theinvention excels as follows:

-   -   the energy beam acts on the absorptive component part in several        cycles such that a local temperature maximum that circulates        together with the energy beam and increases from cycle to cycle        and a correspondingly circulating, locally raised clamping        pressure are generated along the area of joining;    -   the transmissive component part is continuously heated by the        absorptive component part in the area of joining, corresponding        to the circulating temperature maximum;    -   the welding process then takes place by plasticization of the        two component parts in a circulating area of plasticization that        is defined in the longitudinal direction of the weld seam,        equally corresponding to the circulating temperature maximum;        and    -   both component parts are interlocked by a clamping device which        produces a minimum clamping power.

The term “area of plasticization” is to be understood as a sector of thearea of joining between the two parts being joined, where significantwelding of the parts being joined takes place by reason of their degreeof plasticization.

For putting this method into practice, an apparatus for irradiationwelding of two thermoplastic components is comprised as follows:

-   -   a beam source and beam guide for production and guidance of an        energy beam, in particular a laser beam, towards the absorptive        component part;    -   a beam deflection unit for displacement of the energy beam in        cycles along the area of joining such that a local temperature        maximum that circulates together with the energy beam and        increases from cycle to cycle and a correspondingly circulating,        locally raised clamping pressure are generated along the area of        joining; and    -   a clamping device which interlocks the two component parts by a        minimum clamping power.

As will be described in detail below, the circulating local temperaturemaximum which rises from cycle to cycle accompanied with acorrespondingly circulating welding process can be handily termed“tumble welding”. In contrast to the quasi simultaneous weldingaccording to U.S. Pat. No. 6,444,946 B1, the effect of tumble weldingresides in not keeping the temperature along the weld seam as uniform aspossible, but in producing spatial temperature gradients along the weldseam and, by repeatedly scanning, continuously to raise the lowest aswell as the highest temperature along the weld seam. Nevertheless, indoing so, a setting motion of the two parts being joined can be obtainedby circulating, “tumbling” melting of the absorptive part.

It differs from contour welding substantially by repeated scanning ofthe weld seam contour at high feed velocities (for example 500 mm/s)than in contour welding (typical feed 30 mm/s) and by the use of aninterlockable clamping device.

The desired spatial temperature gradients in tumble welding are obtainedby slower feed velocities (for example 500 mm/s) than in quasisimultaneous laser welding (for example typically 3000 mm/s), withreally necessary feed velocities for “tumble welding”, as well as forquasi simultaneous welding, strongly depending on the individualcircumstances, such as combination of material and/or joining geometry.In any case, the plastic material under the focus is heated veryrapidly, temperature increase being higher than in case of the rapidfeed velocities of quasi simultaneous welding, because the time ofirradiation of a certain volume element of the parts being joined isprolonged, it being correspondingly possible to input more energy.Furthermore, the area of the weld seam located ahead of the laser focusin the feed direction is not radiated for a longer time interval than isthe case in quasi simultaneous welding. Therefore, as the currentlyradiated area heats up, in tumble welding a high temperature gradientbetween the currently radiated area and the area ahead thereof in thedirection of feed is generated. Another result consists in the clampingpressure rising locally due to thermal expansion in combination with theobstruction to expansion by the clamping technique.

Proceeding from the relationship of the linear thermal expansion ofstructures, the heat profile of the process, which describes the marchof temperature over the longitudinal coordinate of the weld seam, willdirectly result in a corresponding profile of height of the weldcontour, provided the thermal expansion is not obstructed and thematerials are not yet in a plastic condition. In case of a closed weldseam, this profile of height, together with the laser focus, wouldcirculate around the weld seam, which corresponds to a “tumbling”profile of height.

If the thermal expansion of the absorptive part is counteracted by thethermal part being pressed against it, the inferior rigidity andsolidity of polymers (as compared to steel and the aluminum material ofthe machine frame) will give rise to the assumption that there is nearlyno expansion. However, this results in a locally strong increase ofclamping pressure that circulates along with the temperature maximum.For this local increase of clamping pressure to be produced, any lift ofthe two clamping arrangements that clamp the two joining parts must beprecluded. To this end, the welding system according to the inventionmust not permit any continuous motion of the clamping plate (by contrastto what is said in U.S. Pat. No. 6,444,946 B1). According to theinvention, the clamping arrangement is interlocked in position by acertain clamping power, for example by the locking action of a shoebrake or locking bolt mechanism, or preferably by an electric orpneumatic positioning drive.

The special properties of tumble welding help reduce or avoid therestrictions of quasi simultaneous welding and contour welding:

-   -   Due to the obstructed expansion of a work, locally defined, high        clamping pressure can build up without load excessively acting        on the entire component. Additionally, this local excess of        clamping power, combined with the locally raised temperature,        enables the two surfaces of contact to match rapidly. This helps        create a base for the necessary thermal conduction to the top        layer very early in the course of the process (for example after        a single or two welding cycles). Since thermal conduction is of        extraordinary importance in the build-up of the integral joint,        the weld seamed assembly can be produced gently and efficiently.    -   Since, in tumble welding, as opposed to quasi simultaneous        welding, the field of clamping pressure along the weld seam        changes permanently in the course of the process, the current        local clamping pressure is a characteristic of the local        condition of the process. Consequently, any deviation from the        desired clamping pressure at a certain spot (for example        determined by reference weldments) may signal a single        interruption of the weld seam at this spot (for example by a        notch). The preferred, locally resolved detection of the        clamping pressure along the weld seam can therefore signal        locally defined flaws within the weld seam as opposed to the        weld seam-run monitoring described in U.S. Pat. No. 6,444,946        B1. Detecting the clamping pressure can take place preferably by        the aid of locally resolved pressure measuring films, individual        pressure sensors or by strain gages or load cells in combination        with segmented clamping elements. If inferior lift of the        clamping technique is rendered possible by the clamping frame        pressing by inferior spring load on the parts being joined, then        the tumbling motion of the clamping frame can be measured by        three position measurement sensors (tactile or for example laser        triangulation), because the spatial position of the plane is        defined by three points. This is also how the process can be        characterized. If necessary, the tumbling clamping frame can be        arrested in the course of the process.

The detection of local pressure information can also be used forcontrolling the welding process. The process may for instance be stoppedwhen there is no longer any clamping pressure between the interlockedclamping plates (weld seam completely plasticized).

-   -   In tumble welding, the non-plasticized areas of the absorptive        part being joined prevent the parts being joined from moving        plane-parallel towards each other. Rather, not every spot of the        top-layer weld seam has the same vector of velocity at any time;        the “tumbling” motion results in a fluctuating (partially coming        to a halt) setting motion of the top layer. Nevertheless there        is some inferior setting motion by the circulating melt spew        owing to the locally excessive clamping pressure. In light of        the fact that, in tumble welding, thermal expansion is mainly        used for the production of the clamping pressure and sectional        areas of the seam are still below the plasticization threshold,        the entire melt spew and this setting motion are inferior to        quasi simultaneous welding. Consequently, the delivery of energy        together with the displaced polymer compound is by far lower.    -   As opposed to U.S. Pat. No. 6,444,946 B1, it is the aim of        tumble welding, after individual radiation, to let the area of        the weld seam of the absorptive joining part that is in contact        with the top layer cool down as closely as possible to the        temperature level prior to the radiation for as much energy as        possible to be transported into the top layer. This is intended        to minimize the difference in temperature of the bottom and top        layer in order to avoid any thermal decomposition of the        material, in particular of the bottom layer.    -   Due to the higher linear energy, plasticization of the        absorptive bottom layer is achieved locally defined and more        rapidly in tumble welding than in quasi simultaneous welding. In        connection with the locally excessive clamping pressure, molten        polymer compound can be displaced very rapidly into notches        ahead of the laser focus (seen in the direction of feed), these        notches thus being closed. Once such a flawed spot has been        closed, the necessary thermal conduction to the top layer can        start. The process of tumble welding is more tolerant of flaws        than quasi simultaneous welding. As compared to contour welding,        this effect helps bridge even major joining gaps.

Further features, details and advantages of the invention will becomeapparent from the ensuing description of various embodiments of tumblewelding systems and details thereof, taken in conjunction with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a welding apparatus;

FIG. 2 is a temperature-time-comparison diagram of quasi simultaneouswelding and tumble welding;

FIG. 3 is a diagrammatic side view of a first embodiment of a clampingdevice including the two parts to be weld seamed; and

FIGS. 4 and 5 are a diagrammatic plan view and a sectional view,respectively, on the line V-V of FIG. 4 of a second embodiment of aclamping device.

DESCRIPTION OF PREFERRED EMBODIMENTS

As seen in FIG. 1, a tumble welding system 1 includes a laser beamsource 2 in the form of a diode or Nd-YAG laser of a wave length of 500to 1500 nm. Suitable optics 3 form the laser beam 4 and lead it to ascanner 5. Two mirrors 6, 7 are provided therein at an angle relative toeach other; each of them is adjustable about a pivoting axis bycorresponding positioning motors 8, 9. Thus the laser beam 4 isconventionally deflectable in any directions in space.

By the positioning motors 8, 9 being correspondingly triggered, thelaser beam 4 can be directed by the scanner via focusing optics 10 tothe component parts to be weld seamed 11, 12. The bottom part 11 is madeof laser-irradiation absorptive thermoplastic material, whereas the part12 that constitutes the top layer is laser-irradiation transmissive. Inthe way mentioned at the outset, irradiation welding will take place byheating and melting of the bottom part and corresponding thermalconduction into the top part, accomplishing a weld seamed assembly ofmolten material of the two parts 11, 12 along a weld seam S as seen forexample in FIG. 4.

By the aid of the scanner 5, the laser beam 4 passes across thecomponent parts to be weld seamed 11, 12, circulating in several cyclesalong the area of joining at a feed velocity of approximately 500 to 600mm/s.

FIG. 2 is a qualitative illustration of the march of temperature, ascompared to time, in a volume element of the part 12 that constitutesthe top layer in the vicinity of the area of joining. The volume elementabsorbs a negligible quantity of radiation and is heated exclusively byheat conduction from the bottom part 11. A dashed line in FIG. 2illustrates the march of temperature in a simulated quasi simultaneouspolycarbonate welding process at 3000 mm/s and 80 Watt laser power. Thesolid line reflects a correspondingly simulated tumble welding processat a feed velocity of 600 mm/s, using otherwise identical laserparameters and materials. The dotted line in FIG. 2 is a free-handillustration of the qualitative march of temperature in thelaser-absorptive part 11, attention being drawn to the fact that theabsolute temperature there may be clearly higher—by more than 100°C.—than the temperature of the transmissive top layer part 11. Eachlocal maximum of the dotted march reflects the circulating temperaturemaximum T_(max), with the absolute amount of this maximum rising fromcycle to cycle. On the whole FIG. 2 shows that, in tumble weldingaccording to the invention, the top-layer volume element under regard isheated more rapidly by the time δt so that the melting temperature T_(S)is reached more rapidly.

As for the dashed and solid simulation curve of FIG. 2, the followingformula has been discretized and solved iteratively${\rho \cdot c_{P} \cdot \frac{\partial T}{\partial\quad}} = {{{div}\quad\left( {\lambda \cdot {gradT}} \right)} + {\overset{.}{q}}_{V}}$

-   specifying-   ρ: density-   c_(P): specific thermal capacity    $\frac{\partial T}{{\partial t}\quad}\text{:}$-   temporal temperature modification-   δ: thermal conductivity-   gradT: gradient temperature-   {dot over (q)}_(v): volume heating power.

The formula describes the temporal temperature modification of astructure with internal energy sources and thermal conduction.

The delivery of energy into the bottom layer (component part 11) isincreased by the raised heat conduction in tumble welding, but it isonly the heat flux into the top layer (component part 12) that isdecisive for the formation of a weld seam, because the desired integraljoint can only be attained after melting of the top layer.

In conclusion, the raised heat flux into the top layer (component part12) in combination with the decreased delivery of energy by melt spewconsiderably increases the process efficiency as compared to the methodof U.S. Pat. No. 6,444,946 B1.

FIG. 3 illustrates a clamping device for the component parts 11, 12which is designated by 13 in its entirety. Fundamentally, a stationarytop clamping plate 14 and a bottom clamping plate 16 are used forclamping, the bottom plate 16 being displaceable towards the top plate14 on a linear guide 15. By way of a work carrier 17, the bottomclamping plate 16 holds the absorptive bottom part 11. A clamping frame19 is mounted on the top clamping plate 14, floating by way of springelements 18; the clamping frame 19 fixes the transmissive top part 12.This floating way of mounting is very rigid and does not allow any majormotion, ensuring obstruction to expansion and thus increase of clampingpressure. It is only necessary to show the “tumbling” i.e., the field ofclamping pressure of the top clamping plate that circulates togetherwith the laser. To this end, feedback measurement sensors 20 are placedbetween the clamping frame 19 and the top clamping plate 16, detectingany slight deviation of the clamping frame 19 from a given position inparallel to the plane of the clamping plate 14.

By alternative to the above construction, the circulating field ofclamping pressure can also be detected by a locally resolvedclamping-pressure film that is integrated into the clamping system.

By way of a positioning drive in the form of a toggle mechanism, thebottom clamping plate 16 is displaceable towards the top clamping plate14 by means of a piston-cylinder drive 22 as an actuator. The togglemechanism 21 and the piston-cylinder drive 22 are mounted on a standmember 23.

For tumble welding of the two component parts 11, 12, the bottomclamping plate 16 is moved towards the top clamping plate 14 by a givenclamping power by the aid of the piston-cylinder drive 22 and thenblocked in the closed position by the aid of the interlocking unit 24.The interlocking unit 24 comprises a rack 25 on the piston rod 26 of thepiston-cylinder drive 22 and a locking bolt 27, the engagement of whichwith the rack 25 precluding any displacement within the positioningdrive of the bottom clamping plate 16.

As explained in the introductory part, a locally defined, circulatingclamping-pressure maximum forms in the area of joining between the twoparts 11, 12, owing to the local expansion of the parts conditioned bymelting, this clamping-pressure maximum providing for corresponding,though strictly limited, tumbling motion of the clamping frame 19. Thismotion is detected by the feedback measurement sensors 20 and, being ameasure for the circulating clamping pressure maximum, can be used as aparameter for the control of the welding process by a control unit (notshown in detail).

FIGS. 4 and 5 illustrate a further development of the clamping devicedesignated by 13′ in the vicinity of the top clamping arrangement.Instead of the clamping plate, it comprises a framework 28, on whichindividual clamping segments 29 are mounted by way of pivoted levers 30;they run in accordance with the weld contour S, which is outlined by adashed line in FIG. 4. The clamping segments 29, in their entirety,serve as a back-up for the transmissive top part 12. The pivoted levers30 are double-armed, with the arm opposite the respective clampingsegment 29 acting on a force sensor 31 which detects the force appliedto the clamping segment 29 and transmitting it via a bus line 33 to acorresponding control unit 32. The force sensors 31 are coupled with thebus line 33.

By the actuation force of the clamping segments 29 being detectedindividually, the locally prevailing clamping force of the clampingdevice 13′ can be detected. As explained at the outset, any notches ofthe bottom part, flaws of the weld seam and similar deficiencies can bediscovered in this way.

1. A method of irradiation welding of two thermoplastic component parts(11, 12) by production of a weld seam (5) in an area of joining betweenthe absorptive and transmissive component parts (11, 12) by means of anenergy beam, in particular laser beam (4), wherein the energy beam (4)acts on the absorptive component part (11) in several cycles such that alocal temperature maximum (T_(max)), which circulates together with theenergy beam (4) and increases from cycle to cycle, and correspondinglycirculating, locally increased clamping pressure are generated along thearea of joining; wherein the transmissive component part (12) iscontinuously heated in the area of joining by the absorptive componentpart (11), corresponding to the circulating temperature maximum(T_(max)); wherein a welding process then takes place by plasticizationof the two component parts (11, 12) in a circulating area ofplasticization that is defined in the longitudinal direction of a weldseam, equally corresponding to the circulating temperature maximum(T_(max)); and wherein the two component parts (11, 12) are interlockedby a clamping device (13, 13′) that produces a minimum clamping power.2. A method according to claim 1, wherein a feed velocity of the energybeam (4) ranges between approximately 200 mm/s and 1000 mm/s, preferablybetween approximately 500 mm/s to 600 mm/s.
 3. A method according toclaim 1, wherein the clamping pressure (F) of the clamping device (13,13′) that acts on the two component parts (11, 12) is detected, locallyresolved by way of the clamping length, for one of monitoring andcontrol of the welding process.
 4. A method according to claim 1,wherein the clamping force of the clamping device (13, 13′) is variablyadjustable.
 5. An apparatus for irradiation welding of two thermoplasticcomponent parts (11, 12) by production of a weld seam (5) in an area ofjoining between the absorptive and transmissive component parts (11,12), comprising a beam source (2) and a beam guide (3) for productionand guidance of an energy beam, in particular laser beam (4), towardsthe absorptive component part (11) through the transmissive componentpart (12); a beam deflection unit (5) for displacement of the energybeam (4) in cycles along the area of joining, corresponding to the weldseam, such that a local temperature maximum (T_(max)), which circulatestogether with the energy beam (4) and increases from cycle to cycle, andcorrespondingly circulating, locally increased clamping pressure aregenerated along the area of joining; and a clamping device (13, 13′)that interlocks both component parts (11, 12) by a minimum clampingpower.
 6. An apparatus according to claim 5, wherein the clamping device(13) for each of the two component parts (11, 12) comprises a clampingarrangement (14, 16), namely a stationary clamping arrangement (14) anda positionable clamping arrangement (16), the positioning drive (22) ofwhich is blockable by an interlocking unit (24).
 7. An apparatusaccording to claim 5, wherein at least one clamping arrangement (14) isprovided with a floating, spring-loaded clamping frame (19), a positionof which in space is detectable by a position-sensor unit (20).
 8. Anapparatus according to claim 5, wherein at least one clampingarrangement (28) is provided with a segmented clamping frame (19′), eachclamping segment (29) being coupled with a force sensor (31) fordetection of the clamping force that prevails locally in the vicinity ofthe respective clamping segment (29).
 9. An apparatus according to claim5, wherein the interlocking unit (24) comprises one of a braking andlocking-bolt mechanism (27).
 10. An apparatus according to claim 5,wherein the interlocking unit is designed to combine with thepositioning drive as one of an electric and pneumatic positioningsystem.